Drive about 100 miles east-northeast of San Diego and you’ll come to the Salton Sea, a quasi-oasis whose surface is so glassy it reflects the sky in exquisite detail. Don’t be fooled by the serenity. You’re looking at a potential killer. Beneath the seafloor lie strands of the southern San Andreas fault, a 340-mile system that could rupture all the way to Monterey County. The result would be the “Big One,” an earthquake that experts said would collapse buildings, destroy freeways, warp rail lines and crack dams. Thousands of people could die.